The present invention relates to modem systems, which permit digital data to be efficiently transmitted over an analog channel. A particularly important class of modems is those which transmit data over telephone lines.
The existing telephone network provides such widespread and convenient linkage that it is highly desirable to be able to use it for data transmission. An important objective in such systems is to transmit data at the highest rate possible on the line being used.
In general, modems transmit pulses at a constant rate (the "signalling rate" or "baud rate"), and each pulse carries one of several "symbols." For example, if pulses are sent at 2400 pulses per second, and each pulse corresponds to one of 64 symbols, the resulting data rate will be 14,400 bits per second. (This is commonly written as 14400 bps, or 14.4 kbps.) That is, selecting one of 64 possible symbols is equivalent to specifying 6 bits of data, since 64=2.sup.6.
The set of available symbols is defined by whatever transmission protocol is being followed. Specifically, each symbol corresponds to a specific phase and amplitude value, with reference to a synchronous carrier. (This relation will be discussed in more detail below.) The set of all possible symbols is referred to as the "constellation." An example of a constellation is shown in FIG. 4.
Thus, to change the data rate, it is necessary to change the constellation, change the signalling rate, or both.
Changing the constellation is simpler than changing the signalling rate. When a pair of modems begin communications, one modem will transmit a standard sequence of patterns, which allows the other modem to "tune in" to the constellation which will be used. (This period is called the "training" period.) The last part of this sequence of patterns includes a code which specifies which constellation will be used.
Thus, for example, a change from 14400 bps to 9600 bps could be accomplished by changing constellations, without varying the signalling rate. In fact, a modem can be commanded remotely to make such a change. This is very advantageous, because it permits modems to shift speeds during a communication session.
Each modem normally includes a filter which is constantly watching for a strongly periodic signal. This filter will promptly detect when a training sequence is incoming, and initiate reception of the training operation. The last part of the training sequence will specify which constellation to use.
Thus, to change to a new constellation, one modem would simply begin to transmit the training sequence, instead of data. The other modem detects the training sequence, and, at the completion of this training sequence, the two modems will be able to communicate using the new constellation.
Using a constellation of 64 symbols rather than 16, at a signalling rate of 2400 pulses per second, increases the speed only from 9600 bps to 14400 bps. Similarly, to increase the data rate to 19200 at the same signalling rate, a constellation of 256 symbols would have to be used. Since the signal-to-noise ratio of a telephone line is limited, obtaining the high resolution required for such a constellation may be difficult. (In practice, the constellations actually used are enlarged to permit use of trellis coding. For example, 9600 bps protocols normally use a 32-point constellation, and 14400 bps protocols normally use a 128-point constellation.)
Therefore, for communication at 19200 bps, many modem protocols have slightly increased the signalling rate. Where the number of symbols is large, a small increase in the signalling rate can increase the data rate as much as a doubling of the number of symbols in the constellation. For example, for communication at 19200 bps, increasing the pulse rate by only 14%, from 2400 Hz (=19200/8) to 2742.86 Hz (=19200/7), means that only half as many symbols need be used. If pulses are sent at 2742.8 pulses per second, and each pulse can correspond to any one of 128 symbols, the resulting data rate will be 19,200 bits per second. (In practice, the 19200 bps protocol of the presently preferred embodiment actually uses a 160-point constellation, to permit implementation of coding schemes such as trellis coding.)
However, the use of a higher signalling rate introduces an incompatibility. Remotely initiated speed changes may become more difficult, where some of the protocols used do not have the same signalling rate. For example, a change from 14400 bps to 19200 bps (or vice versa) normally requires a change in the signalling rate.
Previously, modems have had much greater difficulty in detecting data-rate changes where a signalling rate change was necessary. Since most 19200 bps modems use an increased signalling rate, this difficulty has meant that most 19200 bps modems were not able to change speed reliably in response to a command from a remote modem.
A key objective in many telephone-line modem applications (and in other modem types as well) is to maximize the net data rate. However, the maximum possible data rate is limited by the characteristics of the channel. For example, the frequency bandwidth of a telephone line (in the United States) is typically only about 3,000 Hz, and the signal-to-noise ratio is also severely limited. This channel quality is adequate for voice transmission, but makes it difficult to achieve a high data transmission rate. From the Shannon theorem, the absolute theoretical maximum data rate which could fit within the minimum bandwidth and worst-case signal-to-noise standards for dial-up telephone lines (in the United States) would be about 30,000 bits per second ("bps"). However, this is a theoretical limit, which cannot be readily achieved in practice. Moreover, telephone connection quality will vary; some connections will be better than the minimum standard, and some will be worse.
The ability to remotely initiate a speed change, without requiring long training times or introducing significant error rates, is very desirable in telephone line modems. Since the transmission quality of telephone lines varies from line to line, and from minute to minute, it is highly desirable that the modem link should be able to adjust to these variable conditions. This is particularly desirable at higher maximum transmission rates, since a modem which is able to exploit a very good connection (at 19.2 or even 38.4 kbps) must be able to "fall back" to a much lower data if conditions worsen. Similarly, if such a modem has had to operate at a lower rate than its maximum, it is advantageous if the modem can "fall forward" (change to a higher transmission rate) if conditions improve.
It should be noted that not every idea discussed in the foregoing Background of the Invention section of the present application is necessarily prior art. For example, the discussion of technical alternatives may be colored by knowledge of some of the inventive concepts and their advantages. Moreover, some of the technical alternatives discussed may not be "prior art" under the patent laws of the United States or of other countries.
Similarly, the following Summary of the Invention section of the present application may contain some discussion of prior art teachings, interspersed with discussion of generally applicable innovative teachings and/or with specific discussion of the best mode as presently contemplated. Statements made in the Summary section do not necessarily delimit any of the various inventions claimed in the present application or in related applications. Moreover, some statements made in the Summary section may apply to some inventive features but not to others.